The use of plasticizers to extend and soften rubber is an old and well-known technique. Many plasticizers have been used for improving the processing or lowering the cost of rubber compounds. Specialty rubbers, such as neoprene, nitriles, chlorinated and chlorosulfonated polyethylene, polyacrylates, polyurethanes, and polyepichlorohydrins, present a special problem when a suitable plasticizer is desired. A specialty rubber, which is normally more costly, is most often selected because of a desire for better high temperature performance, better solvent and/or oil resistance, and/or better ageing characteristics or physical properties. Incorporation of polar functions into the molecular structure of the rubber is responsible for the enhancement of the desired characteristics. While the presence of these polar functions tends to improve the hydrocarbon solvent resistance and/or the high temperature performance of a particular rubber, it also introduces undesirable effects on the low temperature properties by raising the glass transition temperature, and rendering the polymer brittle. To counteract this and improve the low temperature performance a plasticizer may be compounded with the rubber. As the plasticizer is an extractable and relatively volatile component of the mixture, its addition reduces the solvent resistance and high temperature performance of the rubber, but to obtain reasonable low temperature performance it has been necessary to compromise high temperature performance and solvent resistance.
Because of a growing need for high performance materials in mechanical applications evidenced by higher engine operating temperature for automobiles and machinery as well as use of fuels with increased solvent power, and the extension of areas of use into both the Arctic and Tropics, there has been a continual search for plasticizers which will broaden the working range of specialty rubbers.
Some of the parameters which have been suggested for use in matching a plasticizer and a rubber include, the solubility parameter, the hydrogen bonding effect, free volume of solvents, and the glass transition temperature. A number of statistical and thermodynamic theories have been proposed to account for the effects of a plasticizer in a rubber formulation. While these are useful in selecting particular categories of compounds as likely candidates, no compound can be predicted as particularly useful until it has been treated in actual formulations.
Among the variables of potential plasticizers it is known that polarity, aromaticity, and molecular weight are of importance. This is particularly true of polarity in the case of the specialty rubbers, which contain polar functions and require that a plasticizer have polarity so as to match the solubility parameter and hydrogen bonding effects. Among the more common polar groups used in plasticizers are halogens, nitriles, ethers, esters, and urethanes. Aromaticity is a factor as high aromaticity favors compatibility, in general lowers volatility but normally gives poorer low temperature properties. Molecular weight, of course, affects volatility but high molecular weight compounds in general show lowered compatibility and tend sometimes to bleed out of a particular system at elevated temperatures.
Although many varied plasticizers have been and are used in formulations with the specialty rubbers, desirable low temperature properties, resistance to extraction by solvents, and low volatility are not possessed, to the maximum desired extent, by any.
Di-(butoxyethoxyethanol)adipate is known as a plasticizer, and a polyether plasticizer derived from butoxyethoxyethoxyethanol and thiodiglycol is disclosed in U.S. Pat. No. 3,163,620. The present compounds are polyether esters of selected dibasic acis and they provide significantly better use properties.